Purpose: :
When epiretinal prosthesis array electrodes are placed close to the retina, the strongest electric fields are developed near the nerve fiber layer which could injure astrocytes and Muller cells. We examined the effects of different charge densitiy stimulus pulse trains by epiretinal electrodes on retinal glia cells.

Methods: :
We used a superfused rabbit retinal eyecup prepation for imaging and stimulation. Transparent saline-filled tube electrodes (Cohen et al., J. Neur Eng 2011) were used to deliver biphasic 1msec/phase stimulus pulses to the rabbit retina at 50Hz for periods of 5min. We stimulated retinal regions, with pulse trains of 132, 223, 442, and 749 uC/Cm2/ph. Before stimulation, each electrode was positioned close to the retinal surface using optical coherence tomography (OCT). The location of each stimulus electrode position on the retina was recorded. After test pulse train electrode stimulation and time-lapse OCT imaging under the electrode at 4 locations, the retinal wholemount was immunocytochemically processed using antibodies (Ab) against glutamine synthetase, which selectively labels Muller cells and astrocytes. Stimulated retinal regions were compared between their OCT images, and confocal microscope reconstructions of the Ab-stained regions in retinal wholemounts. We examined glial immunoreactivity under epiretinal stimulus electrodes at 25 different retinal locations.

Results: :
At the lowest charge density pulse train tested 132uC/cm2/ph, there was no damage to the glial immunoreactivity under any of the tested regions (0/5). Similarly OCT cross sectional images showed these stimulus pulse trains had little effect on the retinal layers. Little damage was also seen with 233uC/cm2/ph trains where only one lesion was observed (1/7 regions). However at pulse train charge densities of 442uC/cm2/ph, all stimulated regions showed circular lesions of damage at the inner limiting membrane suggestive of a loss of Muller endfeet immunoreactivity (7/7 regions). The ring-like lesions averaged 469+/-142um (mean s.d.) in diameter. However at 442uC/cm2/ph, Muller cell body immunoreactivity survived under the center of the electrode. With pulse train densitites of 749uC/cm2/ph. a ring-like pattern of disruption could be seen in the Muller cell endfeet and astrocyte fibers under the stimulated region that averaged 521+/-105um in diameter (6/6 regions), and also a disruption of the Muller cell bodies.

Conclusions: :
We found stimulus pulse trains by epiretinal electrodes of 442-749uC/cm/ph could cause significant damage to retinal Muller cell endfeet and astrocytes.